The present invention relates to a light-emitting thyristor whose luminous efficiency is improved and a self-scanning light-emitting device using such light-emitting thyristors.
A surface light-emitting thyristors has been disclosed in the Japanese Patent Publication No. 2-14584, and an end-surface light-emitting thyristor in the Japanese Patent Publication No. 9-85985. The fundamental structure of a surface light-emitting thyristor and that of a end-surface light-emitting are substantially the same, and AlGaAs (Al composition is 0.35, for example) layers are epitaxially grown on a GaAs buffer layer formed on a GaAs substrate, for example.
FIG. 1 is a schematic cross-sectional view depicting a fundamental structure of a light-emitting thyristor. As shown in FIG. 1, on a p-type GaAs substrate 10 successively stacked are a p-type GaAs buffer layer 12, a p-type AlGaAs layer 14, an n-type AlGaAs layer 16, a p-type AlGaAs layer 18, and an n-type AlGaAs layer 30. On the AlGaAs layer 20 provided is a cathode electrode 22, and on the AlGaAs layer 18 a gate electrode 24. An anode electrode 26 is provided on the bottom surface of the GaAs substrate 10.
In this example, a p-type layer, an n-type layer, a p-type layer, and an n-type layer are stacked in this order on a p-type GaAs substrate via a buffer layer. However, an n-type layer, an p-type layer, an n-type layer, and a p-type layer may be stacked in this order on an n-type GaAs substrate via a buffer layer, in this case the uppermost electrode is an anode one, and the bottommost electrode is a cathode one.
In the above-described publications, the inventors of this application have already disclosed a self-scanning light-emitting device structured by arranging such light-emitting thyristors in an array, a self-scanning function thereof being implemented by providing a suitable interaction between neighbored thyristors in the array. The publications have further disclosed that such self-scanning light-emitting device has a simple and compact structure for a light source of a printer, and has smaller arranging pitch of thyristors in the array.
In the light-emitting thyristor having such structure described-above, Al composition is largely varied, for example from 0 to 0.35, at the interface between the GaAs buffer layer and the AlGaAs layer on the buffer layer. Such rapid variation of Al composition causes the turbulence of lattices or the large variation of energy bands at the interface, while the variation of lattice constants is small. As a result, a lattice-mismatching at the interface become large, thereby causing a dislocation. Also, an energy gap at the interface is increased, so that the deformation of energy bands is made large.
Therefore, for the light-emitting thyristor fabricated by growing the AlGaAs layer on the GaAs substrate interposing the GaAs buffer layer therebetween, there are problems such that a device property is degraded due to the increase of a threshold current and a holding current. This is because lattice deffects due to a lattice-mismatching at the interface between the GaAs buffer layer and the AlGaAs layer are induced, and an unclear impurity level is formed at the interface. There are also problems such that an external. quantum efficiency is decreased, resulting in the reduction of the amount of emitted light. This is because defects which serve as xe2x80x9ccarrier killersxe2x80x9d are generated in the vicinity of the interface.
As shown in FIG. 2 wherein like elements are indicated by like reference numerals used in FIG. 1, an n-type GaAs layer 28 may be provided on an n-type AlGaAs layer 20 in a conventional light-emitting thyristor. In this manner, GaAs is used as the material of the uppermost layer for the facility of making ohmic contact with an electrode and the simplicity of material. Since the wave length of emitted light is about 780 nm, the light is absorbed during passing through the uppermost layer (GaAs layer) 28 so that the amount of light to be emitted is decreased.
In order to reduce the light absorption by the GaAs layer 28, the thickness of the layer is needed to be thinner. However, if the layer is thinner, additional problems are caused. That is, alloying of electrode material and GaAs by a heat processing is required to from an ohmic electrode, and atoms migrate for a long distance during the heat processing, as a result of which the alloyed area of electrode material is reached to the AlGaAs layer 20 under the GaAs layer 28. This causes the turbulence of crystalline of AlGaAs, resulting in the scattering of light.
FIG. 3 is a graph showing a light absorption spectrum of an n-type GaAs layer at 297K, wherein ordinate designates an absorption coefficient xcex1 and abscissa a photon energy. The amount of absorbed light is represented by the following formula.
1xe2x88x92exe2x88x92xcex1t (t; film thickness)
It is noted from this graph that the absorption coefficient for the light of 780 nm wave length is about 1.5xc3x97104. Assuming that the film thickness xe2x80x9ctxe2x80x9d is 0.02 xcexcm, it is understood that the amount of emitted light is decreased by 3-4% by calculating the amount of absorbed light based on the above formula. The amount of absorbed light will be further reduced, if the turbulence of atomic arrangement is caused due to the fluctuation of film thickness and the alloying, and the variation of composition.
FIG. 4 shows a light-emitting thyristor in which a GaAs buffer layer 12 is provided on a GaAs substrate 10, and a GaAs layer 28 is used as a topmost layer. In the figure, like element are indicated by like reference numerals used in FIGS. 1 and 2.
In general, a light-emitting thyristor having a pnpn structure is considered to be the combination of a pnp transistor 44 on the substrate side and an npn transistor 46 on the opposite side to the substrate, as shown in FIG. 5. An anode corresponds to an emitter of the pnp transistor 44, a cathode an emitter of the pnp transistor 46, and a gate a base of the pnp transistor 46, respectively. The holding current of the thyristor is determined by the combination of current amplification factors of respective transistors 44 and 46. In order to decrease the holding current, it is required to increase current amplifying factors xcex1 of respective transistors. A current amplifying factor xcex1 is given by the multiplication of an emitter injection efficiency xcex3, a transport efficiency xcex2, a collector junction avalanche multiplication factor M, and a specific collector efficiency xcex1*. In order to increase an emitter injection efficiency xcex3, the impurity concentration of the emitter is designed to be higher that of the base.
The diffusion speed of Zn which is a p-type impurity is very fast, so that Zn is diffused into an n-type semiconductor layer to compensate an n-type impurity. Therefore, if Zn concentration of the anode layer (the GaAs layer 12 and the AlGaAs layer 14) is higher than Si impurity concentration of the n-type gate layer (the AlGaAs layer 16), then most of Si in the vicinity of the interface between the anode layer and the gate layer is compensated to decrease a transport efficiency xcex2 of the transistor. Also, non-luminescent center is generated, causing the reduction of the luminous efficiency of the thyristor.
An object of the present invention is to provide a light-emitting thyristor in which the luminous efficiency thereof is improved, the thyristor being fabricated by growing AlGaAs layers on a GaAs buffer layer formed on a GaAs substrate.
Another object of the present invention is to provide a light-emitting thyristor using GaAs for the material of the uppermost layer, in which the luminous efficiency thereof is improved.
Still another object of the present invention is to provide a light-emitting thyristor including Zn impurity in an n-type gate layer, in which the luminous efficiency thereof is improved.
A further object of the present invention is to provide a self-scanning light-emitting device using such light-emitting thyristors.
An aspect of the present invention is a light-emitting thyristor in which a p-type AlGaAs layer and an n-type AlGaAs layer are alternately stacked to form a pnpn structure on a GaAs buffer layer formed on a GaAs substrate. Al composition of the AlGaAs layer on the GaAs buffer layer is increased in steps or continuously.
According to this light-emitting thyristor, an Al composition of said AlGaAs layer is gradually varied, so that the lattice defects such as dislocation due to lattice-mismatching at the interface between the GaAs buffer layer and the AlGaAs layer may be decreased, and the extreme deformation of an energy band at the interface may be softened.
It is also useful that a single or multi quantum well layer, or a strained superlattice structure is inserted in place of gradual variation of Al composition. In this case, if a quantum well layer or a superlattice layer having a high reflectivity is used, the light toward the substrate is reflected by the quantum well layer or the superlattice layer, thus increasing the amount of emitted light.
When a misfit dislocation is caused in the AlGaAs layer in which Al composition is varied in steps or continuously, a quantum well layer or a strained superlattice structure may be inserted into the AlGaAs layer in order to block the propagation of the misfit dislocation.
According to a second aspect of the present invention, the light absorption by the topmost layer may be decreased by utilizing a material such as InGaP, InGaAsP, or AlGaInP having a absorption edge wave length shorter than 780 nm. It is desirable that this material is lattice matched with the GaAs substrate. As a result, an external quantum efficiency may be increased because the light absorption by the topmost layer is decreased.
According to a third aspect of the present invention, an impurity concentration of at least a part of an anode layer near an n-type gate layer is lower than an impurity concentration of the n-type gate layer in a pnpn structure light-emitting thyristor. Where a p-type first layer, a p-type second layer, an n-type third layer, a p-type fourth layer, an n-type fifth layer, and an n-type sixth layer are epitaxially grown on a p-type substrate, for example, such light-emitting thyristor is composed of the combination of a pnp transistor on the substrate side and an npn transistor on the opposite side to the substrate.
According to the present invention, each impurity concentration of the first and second layer is equal to or smaller than that of the third layer to limit a impurity diffusion from the first and second layer to the third layer. Since an emitter-base junction of a pnp transistor is a hetero junction, even if an impurity concentration of an emitter is lower than that of a base, an emitter injection efficiency is not affected and is held to about 1.
Using light-emitting thyristors described above, a self-scanning light-emitting device of the following structure may be implemented.
A first structure of the self-scanning light-emitting device comprises a plurality of light-emitting elements each having a control electrode for controlling threshold voltage or current for light-emitting operation. The control electrodes of the light-emitting elements are connected to the control electrode of at least one light-emitting element located in the vicinity thereof via an interactive resistor or an electrically unidirectional element, and a plurality of wirings to which voltage or current is applied are connected to electrodes for controlling the light emission of light-emitting elements.
A second structure of the self-scanning light-emitting device comprises a self-scanning transfer element array having such a structure that a plurality of transfer elements each having a control electrode for controlling threshold voltage or current for transfer operation are arranged, the control electrodes of the transfer elements are connected to the control electrode of at least one transfer element located in the vicinity thereof via an interactive resistor or an electrically unidirectional element, power-supply lines are connected to the transfer elements by electrical means, and clock lines are connected to the transfer elements; and a light-emitting element array having such a structure that a plurality of light-emitting elements each having a control electrode for controlling threshold voltage or current are arranged, the control electrodes of the light-emitting element array are connected to the control electrodes of said transfer elements by electrical means, and lines for applying current for light emission of the light-emitting element are provided.
According to the structures described above, an increased luminous efficiency, high-density, compact and low-cost self scanning light-emitting device may be implemented.